it amuses me that most (myself included) are worried about bar slipping on a front squat, yet I know personally I was never worried about it falling off on a back squat! Once you get comfortable you'll see that the bar is very secured.

on a backsquat there is more room for error.you can essentially do a goodmorning to help you get it up. front squat you really have to drive your chest up if you feel like you cant get it. ive failed on a FS and backsquat before.